Ancient Plants / Being a Simple Account of the past Vegetation of the Earth and of the Recent Important Discoveries Made in This Realm of Nature

by Marie Carmichael Stopes

s, Seeds attached on either side below the divisions of the sporophyll.

In all the members of the family, excepting Cycas itself, the female fructifications also consist of similarly organized cones bearing a couple of seeds on each scale instead of the numerous pollen sacs. In Cycas the male cones are like those of the other genera, and reach an enormous size; but there are no female cones, for the seeds are borne on special leaflike scales. These are illustrated in [fig. 75], which shows also that there are not two seeds (as in the other genera with cones) to each scale, but an indefinite number.

The leafy nature of the seed-bearing scale is an important and interesting feature. Although theoretically botanists are accustomed to accept the view that seeds are always borne on specially modified leaves (so that to a botanist even the “shell” of a pea-pod and the box of a poppy capsule are leaves), yet in Cycas alone among living plants are seeds really found growing on a large structure which has the appearance of a leaf. Hence, from this point of view (see [p. 45], however, for a caution against concluding that the whole plant is similarly lowly organized), Cycas is the most primitive of all the living plants that bear seeds, and hence presumably the likest to the fossil ancestors of the seed-bearing types. In this character it is more primitive than the fossil group of the Cordaiteæ, and comes very close to an intermediate group of fossils to be considered in the next chapter.

Fig. 76.—Seed of Cycas cut open

n, The nucellus, fused at the level l to the coat c; sc, stony layer of coat; p.c, pollen chamber in apex of the nucellus; S, “spore”, filled with endosperm, in which lies the embryo e.

To enter into the detailed anatomy of the seeds would lead us too far into the realms of the specialist, but we must notice one or two points about them. Firstly, their very large size, for ripe seeds of Cycas are as large as peaches (and peaches, it is to be noted, are fruits, not seeds), and particularly the large size they attain before they are fertilized and have an embryo. Among the higher plants the young seeds remain very minute until an embryo is secured by the act of fertilization, but in the Cycads the seeds enlarge and lay in a big store of starch in the endosperm before the embryo appears, so that in the cases in which fertilization is prevented large, sterile “seeds” are nevertheless produced. This must be looked on as a want of precision in the mechanism, and as a wasteful arrangement which is undeniably primitive. An even more wasteful arrangement appears to have been common to the “seeds” of the Palæozoic period, for, though many fossil “seeds” are known in detail from the old rocks, not one is known to have any trace of an embryo. A general plan of the Cycas seed is shown in [fig. 76], which should be compared with that of Ginkgo ([fig. 68]). The large size of the endosperm and the thick and complex seed-coats are characteristic features of both these structures. Another point that makes the Cycad seeds of special interest is the fact that the male cells (as in Ginkgo) are developed as active, free-swimming sperms, which swim towards the female cell in the space provided for them in the seed (see p.c, [fig. 76]).

The characters of the Cycads as they are now living prove them to be an extremely primitive group, and therefore presumably well represented among the fossils; and indeed among the Mesozoic rocks there is no lack of impressions which have been described as the leaves of Cycads. There is, however, very little reliable material, and practically none which shows good microscopic structure. Leaf impressions alone are most unsafe—more unsafe in this group, perhaps, than in any other—for reasons that will be apparent later on, and the conclusions that used to be drawn about the vast number of Cycads which inhabited the globe in the early Mesozoic must be looked on with caution, resulting from the experience of recent discoveries proving many of these leaves to belong to a different family.

There remain, however, many authentic specimens which show that Cycas certainly goes back very far in history, and specimens of this genus are known from the older Mesozoic rocks. We cannot say, however, as securely as used to be said, that the Mesozoic was the “Age of Cycads”, although it was doubtless the age of plants which had much of the external appearance of Cycads.

From the Palæozoic we have no reliable evidence of the existence of Cycads, though the plants of that time included a group which has an undoubted connection with them.